Particles flowing serially and randomly through an electrical field constriction produce a series of electrical pulses, which may be detected by commercially available electrozone particle sensing equipment. Electrozone sensing responds to the displacement volume of each particle as the amplitude of its electrical pulse, thus providing the basis for electrozone particle size analysis (PSA) via a frequency histogram. Statistical adequacy (quantitative) is readily achieved.
In automated particle size analysis, the electrozone method has three intrinsic advantages in the elements of precision; true size sensing, low side effects and simplicity. These result in response purity and sensing reliability that make it naturally superior to the other major automated methods, sedimentation and various light beam interference methods including forward scatter, obscuration, diffraction and beam transit time.
All of these PSA methods use a fluid suspension of particles and all require the concentration of particles to be low enough to allow discrete particles mainly to generate the measurement signal, whether derived from a single particle or the massed effect of multiple particles of approximately equal size. With increased concentration, all PSA methods have increased inter-particle interferences, termed "sensing zone coincidence" in sensing zone methods, "concentration effect" in massed effect light scattering methods, "hindered settling" in sedimentation methods. As concentration is increased through the region of significant interference, the distribution histogram becomes increasingly distorted and eventually a state is reached where it is based on virtually no signals generated from discrete particles and is therefore usually useless.
The precision that is possible with the electrozone method provides the highest quality of measurement fidelity and hence a unique possibility for mathematical elimination of coincidence effects that produce unacceptable data distortion.
In the prior art, attempts have been made to correct for coincidence in single particle sensing when using an optical measuring technique, but in optical systems a correction is not practical due to multiple secondary responses of the light beam interference sensors, which seriously compromise the meaning of the pulse amplitudes which are realized. Secondly, the attempt to treat the optically generated pulses deals only with binary coincidence, and does not consider the significant effects of tertiary or higher levels of coincidence. Moreover, previous attempts to compensate for particle coincidences do not recognize the possible degrees of additivity of the component particle signals or the probability of pulses occurring during the dead time following recognized pulses, nor do they attempt to make the coincidence correction on a real time basis, i.e., within one minute.
The electrozone principle has three intrinsic advantages over the sedimentation or light beam interference methods which make it uniquely able to utilize rigorous mathematics for distributional coincidence correction.
In detail, the three aforesaid intrinsic advantages of the electrozone method are:
1. The truest measure of particle size, sensing the displacement volumes of individual particles in an electrical field, with the proportionality being true to a fraction of a percent from less than 0.5 to more than 1000 micrometers.
In light beam interference, particle size is sensed as a projected area "seen" by the light beam. On this basis, the size of a man would be described by his shadow's area without the sun's position being known, instead of his weight. In sedimentation, sensing is based on the surface area subject to fluid drag at the extant particle orientation. In both cases, errors can be enormous. Also, area sensing (D.sup.2) has much lower resolution than volume sensing (D.sup.3).
Further, none of these area responsive methods has a consistent response sensitivity throughout its size range. The response in light beam interference is increasingly poor as size drops below the five to seven micron region, and sedimentation rates may be exceptionally slow for smaller sizes and non-spherical particles by some orders of magnitude.
2. The least side effects in sensing: shape response amounts to a few percent of diameter at most, conductivity response is readily avoided, there are no significant effects from other physical properties of particles or suspension liquids.
Light beam interference methods are greatly affected by shape as well as refractive index, opacity and absorption. Sedimentation is also highly sensitive to particle shape, as well as particle density, optical properties (when using light beams), and to liquid (or gas) density, convection and viscosity (note falling ball or falling needle viscometers).
3. The simplest sensor structure, allowing highest stability and lowest costs. Electrozone employs merely a simple hole vs. a multi-instrument sensor for the said other major methods: beam source, optical train, photic receptor, plus (in sedimentation) a temperature/convection control system, plus (when accelerated) a centrifuge control system.
Electrozone calibration may be general (accurate to within a few percent) or of pin-point precision through easily used standard particles, narrowly distributed. Precise calibration allows accuracy within a few tenths of one percent (stating size as "volume equivalent spherical diameter").
Calibrations for sedimentation and light beam interference methods are done by theoretical response equations or by the use of specific materials having known distributions or by narrowly distributed standards. Data blurring, due to the compromises of side effects and multi-particle response as described above, is evidenced, e.g., by a marked spreading of the distributions for narrowly sized standards. Because of the compromises, these area sensing methods can only be calibrated for specific applications.
In the electrozone method for counting and sizing of individual particles there is inevitable coincidence in the sensing zone. The portion of the total particle population so involved is termed the "particles dedicated to coincidence" (PDC). Its distribution histogram shape is the same as that of the total population.
The total coincidence effect on the entire population is a net loss of particle count. The distributed coincidence effect is the gain and/or loss of count at every increment or channel of particle size. The algebraic sum of the distributed coincidence throughout the population equals the total coincidence.
In the literature, the total count loss is referred to as "primary coincidence", and the pseudo-distribution formed from distributed coincidence is referred to as "secondary coincidence". Pulse shapes formed by various degrees of particle proximity in coincidence have been described (FIG. 1).